*Rio Kastle, known as Rio Kamishiro (神代かみしろ璃緒りお, Kamishiro Rio) in the Japanese version, is one of the main characters, appearing in the Yu-Gi-Oh! ZEXAL anime. She is the younger twin sister of Shark. Rio is later revealed to be Marin (Merag (メラグ, Meragu) in the Japanese version), a member of the Seven Barian Emperors, who became determined to follow her brother no matter what happened to them and sided with the Barian World. She was absorbed by Vector after losing a Duel to him. She was later revived after Yuma and Astral's Duel.

Rio has dark pink eyes and like her brother, she has pale skin and dark blue hair with light blue bangs. Rio also wears a ring on her right little finger, which was previously owned by her brother. Rio's appearance attracts many initial admirers, including Bronk.

Her attire includes a white jacket over a light pink shirt, black stockings, white skirt and shoes (her skirt is also longer, covering the bare skin showing on her legs in the dub). The left coattail of her jacket has a conch-like ornament hanging down. At school, she wears the standard uniform, with hers being green due to her status as a second year student (in the dub, her skirt is lengthened). She wears longer stockings than most female students. During the group's trip to Spartan City, she bought a casual outfit composed of a blue-white striped sleeveless T-shirt, a blue upper knee length skirt and white high-heeled sandals.

Despite her appearance and previously spending a large amount of time in hospital, she is a confident and adamant person, getting annoyed when things do not go her way. This is usually shown when conversing with her brother. Her confident personality assists in her interests which allows her to conquer sports such as soccer, baseball, basketball and boxing with ease. Despite all of this, Rio also shows somewhat of a childish side as well, shown by her fear of cats (this showing she has ailurophobia) and calling Shark a "meanie" when he teasingly refused to help remove one from her sight. Rio tends to be annoyed when people refer to her as "Shark's sister" instead of calling her by name as indicated by Yuma due to her snapping at him when he continually calls her as such, and especially does not like it when someone insults her brother. This is further seen many times during her Duel against Lotus as she got mad at her for making a rude remark about Shark.

Despite being occasionally irritated with Yuma for not addressing her properly as "Rio", his personality and playful antics makes her let it slide given that she is thankful to him for helping her brother and being his friend. It is also revealed that she goes very far on things and acts strong because she feels she has to or people may use her as a shield against her brother. She later shows admiration towards Yuma and Alito as she commented they have amazing Dueling spirit.

She takes great pride in her abilities and effort as she has no problem in showing them when people underestimate her as she believes women are not to be viewed as "pretty decorations".

Rio also has a empathetic quality to her personality as well. This is demonstrated when she finds Bronk injured on the way to school, and shows genuine concern to him and helps treat his injuries.

She has undying loyalty to her brother, going along with his decision to assault Earth after regaining their Barian identities, despite her own wishes.*